Diphenylmethane, also known as 1,1'-methylene-bis-benzene, is useful for the production of heat transfer fluids, aromatic solvents, fragrances, and monomers for polycarbonate resins.
Various processing schemes comprising alkylation and/or transalkylation reactions are known to produce monoalkylaromatic or polyalkylaromatic products (such as diphenylmethane) in high yield. However, one of the issues burdening existing processes is the production of undesirable by-products through secondary reactions. Examples of such by-products produced in conjunction with monoalkylaromatic or polyalkylaromatic products include alkylating agent oligomers, heavy polyaromatic compounds, and polyalkylated compounds.
Another issue burdening existing processes concerns the use of Friedel-Crafts catalysts based on aluminum chloride and other Lewis, Bronsted, or Bronsted-Lewis types of acids; commercial technology associated with Friedel-Crafts catalysis based on aluminum chloride is subject to more than one disadvantage. In such systems, selectivity to desired products in Friedel-Crafts reactions is frequently limited, and complex multicomponent mixtures frequently result from affiliated competitive reactions. Friedel-Crafts catalysts are corrosive, requiring use of expensive metals in plant construction. These catalysts also generally require the use of a water or acid gas co-catalyst which produces a corrosive sludge by-product. These and other problems associated with Friedel-Crafts catalysts can be avoided by the use of organoaluminum Lewis acids with certain alkylating agents.
It is known that aromatic hydrocarbons can be alkylated in the presence of alkylaluminum halide reagents. Such technology is discussed in U.S. Pat. No. 3,787,512 to G. E. Nelson entitled "Preparation of Aromatic Hydrocarbons" which refers to the alkylation of benzene or alkylated benzene by an alkyl halide in the presence of an alkylaluminum halide. This patent is incorporated herein by reference in discussion of the background of this invention. U.S. Pat. No. 3,875,249 to G. E. Nelson describes a "process for alkylating an aromatic hydrocarbon by reacting a haloalkane with either benzene or an alkyl benzene . . . in the presence of an alkylaluminum halide catalyst". More specifically, U.S. Pat. No. 3,787,512 indicates combination of methylene chloride and benzene in the presence of methylaluminum sesquichloride and also methylene iodide and benzene in the presence of ethylaluminum sesquichloride to make diphenylmethane.
The Applicants for the present invention, however, have discovered that the synthesis of diphenylmethane from benzene and methylene chloride in the presence of methylaluminum dichloride or ethylaluminum dichloride affords some clear and distinguishing benefits; these benefits indicate that organoaluminum dichlorides in general have distinct and special usefulness in alkylations which are reactively similar to the focal diphenylmethane synthesis. In this regard, methylaluminum and ethylaluminum dichloride catalysts greatly increase the rate of reaction of benzene and methylene chloride compared to the catalyst technologies utilizing aluminum chloride or the organoaluminum sesquichlorides specifically cited by Nelson in U.S. Pat. No. 3,787,512. In practice, the reaction of methylene chloride and benzene in the presence of either methylaluminum sesquichloride or ethylaluminum sesquichloride in an appropriate reaction environment shows an "induction time" (where essentially no or very little conversion to diphenylmethane is apparent) lasting from 30-60 minutes after initial contact of the reactants. In contrast, and in an unexpected result, Applicants in the present invention discovered that the reaction of benzene and methylene chloride in the presence of either methylaluminum dichloride or ethylaluminum dichloride occurred immediately and rapidly such that the conversion to diphenylmethane was complete in a commercially attractive period of time (for methylaluminum dichloride, .about.75% complete in 15 minutes).
In accordance with the present invention, there has been discovered a process for decreasing the selectivity for undesirable by-products while using an organoaluminum catalyst which affords a high yield of the alkylate of interest over a sustainable and commercially attractive period of time. One particular advantage of the process of the present invention derives from the utilization of alkylation at high temperatures (greater than 100 degrees Celsius for this type of reaction), because a commercially attractive period of time for completion of the reaction is provided even as high selectivity for useful products such as diphenylmethane is achieved.